1. Field of the Invention
The present invention relates to RFID tags which are attachable to products, cards, etc.
2. Background Art
Recently, various usages of RFID tags have been proposed. One proposal is, for example, to attach radio tags to products so that inventory control and distribution management can be done without a need of personnel. Utilizing RFID tags for controlling food in a home refrigerator has also been proposed.
Meanwhile, as usage of RFID tags diversifies, problems associated with it have arisen. A privacy issue discussed below is pointed out as one of such problems. Namely, because information in a RFID tag can be read from places apart from the tag, it is possible that what is inside one's bag is known by a third party or one's behavior is tracked based on information that the tag transmits. In order to solve the aforementioned problem, Patent Document 1, for example, proposes a method for protecting information in a tag by varying communication distance of an active tag.
An active tag disclosed in Patent Document 1 includes a battery for powering the various electronic circuits, amplifying means, a demodulator for demodulating incoming signals and a data processing unit. The demodulator includes validating means for validating the data included in the incoming signals. The data processing unit includes a circuit for controlling maximum communication distance between the tag that transmits signals at a predetermined power and a reader. This circuit transmits signals for varying a gain of the amplifying means or a reference voltage of the validating means to control the maximum communication distance.
Patent Document 1: Japanese Patent Laid-open No. 2001-358611